Freeze-drying technology is a process by which moisture contained in a moisture-containing material is frozen to be discharged after being directly transformed from solid phase into gas phase so as to obtain a dry product, also known as lyophilization. Hot plates or infrared radiation plates are generally used for providing sublimation heat in traditional freeze-drying methods, traditional freeze-drying methods are widely applied to food, medicine and new material processing fields, but severely restrict further development of freeze-drying technologies due to their low drying rate, long cycle, complex equipment, high manufacturing cost and huge power consumption.
Microwave freeze-drying is a new technology combining high-efficiency microwave radiation heating technology and vacuum freeze-drying technology, allows water molecules of the material to vibrate and rub against each other under the action of a microwave field by 3D heating characteristic of microwave, thus converting electric energy into latent sublimation heat for moisture sublimation. Compared with traditional freeze-drying methods, the biggest advantages of microwave freeze-drying include quick drying speed and high thermal efficiency, and the freeze-drying speed is 4-20 times of that of a conventional heating method.
The major technical problem of the microwave freeze-drying technology is glow discharge of microwave under vacuum environment, vacuum pressure for freeze-drying is generally at 1-610 Pa, and the vacuum pressure is the pressure range in which disruptive discharge easily occurs in the microwave field. In practical application, glow discharge of microwave frequently occurs in the freeze-drying bin, resulting in gas ionization in the freeze-drying bin, thus causing harmful material change, effective heating power loss of microwave and freeze-drying failure. No large-scale industrial application of microwave freeze-drying technology is found at home and abroad through literature retrieval.
The Chinese invention patent application 200910181720.1 titled “Double-bin differential pressure microwave vacuum freeze-drying device” with publication No. of 101608862 and publication date of Dec. 23, 2009 discloses a microwave freeze-drying device, a freeze-drying bin thereof is separated into a first bin and a second bin by a transmitting baffle wall, and a magnetron is located in the first bin, with relative vacuum degree smaller than discharge critical value, and a material loading device is located in the second bin and connected with a cold trap by a shield overflow plate. The second bin is also a vacuum space, frequent discharge of microwave still occurs in the space, the method only helps reduce the probability of glow discharge, practices show that the simple method of separating the freeze-drying bin into two bins of different pressures can not fundamentally prevent occurrence of vacuum discharge, can not be used under industrial environment, and has difficulty in implementation.
The Chinese invention patent application 200910059544.4 titled “Continuous microwave freeze-drying system” with publication No. of 101922855A and publication date of Dec. 22, 2010 discloses a continuous microwave freeze-drying device using a combined microwave freeze-drying bin. The invention comprises a microwave vacuum freeze-drying device, a vacuum water-trapping device, a vacuum feed device and a vacuum discharge device, the microwave vacuum freeze-drying device is provided with a combined freeze-drying bin consisting of an atmospheric microwave chamber and a vacuum microwave chamber, the atmospheric microwave chamber is separated from the vacuum microwave chamber by a microwave transmitting vacuum baffle plate, the vacuum microwave chamber and the vacuum water-trapping device is connected by a vapor channel, and the vacuum microwave chamber is separated from the vapor channel by a porous ventilating microwave shield plate. The technology core is to separate the freeze-drying bin into the atmospheric microwave chamber and the vacuum microwave chamber by a microwave transmitting material to solve easy discharge of microwave under vacuum environment. Experiments show that the method of separating the freeze-drying bin into the vacuum bin and the atmospheric microwave bin by a microwave transmitting material reduces the probability of discharge to some extent. However, as the freeze-drying bin is only separated by the microwave transmitting material, that is, one wall of the freeze-drying bin is a metal conducting plate, two walls are metal plates and another wall is of microwave transmitting material, in this way, microwave is reflected by three metal bin walls of the freeze-drying bin under vacuum environment, thus resulting in non-uniform microwave field, non-uniform drying of materials, low product yield and poor quality. Meanwhile, the feed belt and the return belt of the belt conveying system are located in the freeze-drying bin, increasing manufacturing difficulty and cost, and making operational failure rate high and implementation difficult.